Bone marrow transplantation in lethally irradiated mice and irradiated control mice were used to investigate the effects of qualitative differences in irradiation as they affect survival and the reproducibility of experimental data in this biological test system. The direction of the irradiation was an important factor with ventral exposure the least effective with dorsal exposure being less effective than the dual exposure from opposing x-ray tubes. Qualitative differences in the radiation were apparently responsible for the development of partial chimeras and the failure to effectively eliminate preleukemic cells even in young mice. The need for more precise information on the irradiation facilities used in experimentation to be included in published reports is discussed.